Aqueous polymer dispersions, aqueous coating materials based on these polymer dispersions, and processes for the finishing of motor-vehicle bodies

ABSTRACT

An aqueous polymer dispersion, comprising an at least 20% neutralized, free-radical polymerized product of A and B in a ratio of one part A to from 0.5 to 9.0 parts B by weight; wherein 
     A consists essentially of from 2.5 to 13% by weight of at least one acid group-containing, ethyleneically unsaturated monomer and from 87 to 97.5% by weight of at least one ethylenically unsaturated monomer that is free from acid groups, and further wherein 
     B is a reaction product of at least one compound containing at least 1.5 epoxide groups per molecule on average and at least one member of the group consisting of: 
     (1) mixtures of ethylenically unsaturated monocarboxylic acids, saturated fatty acids, and monoesters of ethylenically unsaturated dicarboxylic acids and saturated fatty alcohols; and (2) mixtures of saturated fatty acids and monoesters of ethylenically unsaturated dicarboxylic acids and saturated fatty alcohol.

FIELD OF THE INVENTION

The invention relates to aqueous polymer dispersions, to aqueous coatingmaterials based on these polymer dispersions, and to a process for thefinishing of motor-vehicle bodies.

BACKGROUND AND SUMMARY OF THE INVENTION

Aqueous polymer dispersions and aqueous coating materials based onaqueous polymer dispersions are known. EP-A-469,646, for example,describes aqueous polymer dispersions of hybrid polymers, which are usedfor the production of aqueous coating materials. The hybrid polymers areprepared by polymerizing ethylenically unsaturated monomers in thepresence of an epoxide-containing reaction product of n mol of abisepoxide compound and n-1 mol of a dicarboxylic acid, which producthas itself been reacted with unsaturated fatty acids. Followingneutralization of the acid groups contained in the polymer, the hybridpolymer is dispersible in water. From the aqueous polymer dispersionsprepared in this way it is possible to product aqueous coating materialswhich, disadvantageously, give coating films which have a tendency toyellow, especially when relatively long baking times and/or relativelyhigh baking temperatures are employed.

The object of the present invention is to provide aqueous polymerdispersions from which it is possible to produce aqueous coatingmaterials which can be used to produce coating films in which theabove-described disadvantages are reduced or absent and whichadditionally have good surface properties, in particular a high acidresistance and scratch resistance.

This object is surprisingly achieved by the provision of aqueous polymerdispersions which can be prepared in that

(A) 1.0 part by weight of a mixture of

(a1) from 2.5 to 13% by weight of an ethylenically unsaturated monomerwhich contains acid groups, or of a mixture of such monomers, and

(a2) from 87 to 97.5% by weight of an ethylenically unsaturated monomerwhich is free from acid groups, or of a mixture of such monomers,

the sum of the percentages by weight of components (a1) and (a2) alwaysbeing 100% by weight, are subjected to free-radical polymerization inthe presence of

(B1) from 0.5 to 9.0 parts by weight of a reaction product of

(b1) a compound which contains on average per molecule at least 1.5epoxide groups, or a mixture of such compounds, and

(b2) a mixture of

(b21) an ethylenically unsaturated monocarboxylic acid having 3-8 carbonatoms, or a mixture of such monocarboxylic acids, and

(b22) a saturated fatty acid, or a mixture of saturated fatty acids,

or in the presence of

(B2) from 0.5 to 9.0 parts by weight of a reaction product of component(b1) and

(b3) a monoester of an ethylenically unsaturated dicarboxylic acid and asaturated fatty alcohol, or a mixture of such monoesters,

or in the presence of

(B3) from 0.5 to 9.0 parts by weight of a reaction product of component(b1) and

(b4) a mixture of components (b21), (b22) and (b3) or a mixture ofcomponents (b21) and (b3) or a mixture of components (b22) and (b3).

and the resulting polymerization product is converted, during or afterthe neutralization of at least 20 percent of the acid groups containedin the polymerization product, into an aqueous polymer dispersion,components (b1), (b2), (b3) and (b4) being reacted with one another inproportions such that components (B1), (B2) and (B3) contain on averageper molecule not more than 0.25 epoxide groups and at least 0.4ethylenically unsaturated groups which derive from component (b21)and/or component (b3).

DETAILED DESCRIPTION

As component (a1) it is possible in principle to employ anyethylenically unsaturated monomer which contains acid groups, or amixture of such monomers. Examples of ethylenically unsaturated monomerswhich contain acid groups are acrylic acid, methacrylic acid, maleicacid and fumaric acid. As component (a1) it is preferred to employmethacrylic acid or acrylic acid or a mixture of methacrylic acid andacrylic acid.

As component (a2) it is possible to employ any ethylenically unsaturatedmonomer which is free from acid groups, or a mixture of such monomers.Examples of ethylenically unsaturated monomers which are free from acidgroups are: (cyclo)alkyl (meth)acrylates having 1-12 carbon atoms in the(cyclo)alkyl radical, for example methyl (meth)acrylate, ethyl(meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate,isobornyl (meth)acrylate, dodecyl (meth)acrylate and cyclohexyl(meth)acrylate; (cyclo)alkyl esters of maleic acid, fumaric acid acidand itaconic acid, for example dimethyl maleate, diethyl maleate,diethyl fumarate, dipropyl maleate, dibutyl maleate and dibutylfumarate; (meth)acrylates containing ether groups, for example2-methoxyethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate and3-methoxypropyl (meth)acrylate; hydroxyalkyl (meth)-acrylates, forexample 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,4-hydroxybutyl (meth)-acrylate, 6-hydroxyhexyl (meth)acrylate,hydroxycyclohexyl (meth)acrylate, hydroxypolyethylene glycol(meth)acrylates, and hydroxypolypropylene glycol (meth)acrylates;ethylenically unsaturated aromatic compounds such as styrene,vinyltoluene and α-methylstyrene, and other ethylenically unsaturatedmonomers such as, for example, (meth)acrylamide, (meth)acrylonitrile,N-methylol(meth)acrylamide, vinyl acetate, vinyl propionate andvinylpyrrolidone.

Component (A) is composed of 2.5-13% by weight, referably from 5 to 10%by weight, of component (a1) and 87-97.5% by weight, preferably from 90to 95% by weight, of component (a2), the sum of the percentages byweight of components (a1) and (a2) always being 100% by weight.Components (a1) and (a2) are preferably selected such that thepolymerization of component (A) alone leads to a polymer having an acidnumber of from 23 to 100, preferably from 40 to 70, a hydroxyl number offrom 30 to 200, preferably from 40 to 140, and a glass transitiontemperature T_(G) of from -40 to +70° C., preferably from -20 to +50° C.

Component (B1) is a reaction product of component (b1) and component(b2), which contains on average per molecule not more than 0.25 epoxidegroups and at least 0.4, preferably from 0.5 to 1.8, particularlypreferably from 0.8 to 1.3, ethylenically unsaturated groups derivingfrom component (b21). Component (b1) is preferably free from epoxidegroups.

As component (b1) it is possible to employ any compound which containson average per molecule at least 1.5 epoxide groups, or a mixture ofsuch compounds. The compound which contains on average per molecule atleast 1.5 epoxide groups may be, for example, a diglycidyl ether ofbisphenol A, an epoxide-containing oligomer of epichlorohydrin andbisphenol A having a number-average molecular weight of 300 to 2000, anepoxide-containing oligomer of epichlorohydrin and hydrogenatedbisphenol A having a number-average molecular weight of from 300 to2000, 1,4-butanediol diglycidyl ether, polypropylene glycol diglycidylether, bis(2,3-epoxy-6-methyl-cyclohexylmethyl) adipate and3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate.

As component (b1) it is preferred to employ a reaction product of

(b11) n mol of a compound which contains on average per molecule from1.5 to 2.5 epoxide groups, or of a mixture of such compounds, and

(b12) n-1 mol of a dicarboxylic acid having from 2 to 40 carbon atoms,or of a mixture of such dicarboxylic acids,

where n is 2-10.

As component (b11) it is possible to employ any compound which containson average per molecule from 1.5 to 2.5 epoxide groups, or a mixture ofsuch compounds. The epoxide-containing compounds listed above can beemployed as component (b11).

As component (b12), a dicarboxylic acid having from 2 to 40 carbonatoms, or a mixture of such dicarboxylic acids, is employed. Examples ofdicarboxylic acids having from 2 to 40 carbon atoms are: oxalic acid,adipic acid, isophthalic acid, phthalic acid, hexahydrophthalic acid,sebacic acid, dodecanedioic acid, azelaic acid and dimerized fattyacids.

As component (b1) it is preferred to employ compounds which contain noaromatic structural elements.

As component (b2) a mixture is employed of

(b21) an ethylenically unsaturated monocarboxylic acid having from 3 to8, preferably from 3 to 6, carbon atoms, or a mixture of suchmonocarboxylic acids, and

(b22) a saturated fatty acid or a mixture of saturated fatty acids.

Examples of ethylenically unsaturated monocarboxylic acids which can beemployed as component (b21) are: acrylic acid, methacrylic acid andethacrylic acid.

As component (b21) it is preferred to employ acrylic acid or methacrylicacid or a mixture of acrylic and methacrylic acid. As component (b22) asaturated fatty acid is employed, preferably a saturated fatty acidhaving from 14 to 24, particularly preferably having from 16 to 20,carbon atoms, or a mixture of such saturated fatty acids. Examples ofsaturated fatty acids which can be employed are: myristic acid, palmiticacid, stearic acid, arachidic acid, behenic acid and lignoceric acid.

Component (B1) is prepared by reacting components (b1) and (b2) with oneanother in a proportion such that the reaction product contains onaverage not more than 0.25 epoxide groups and at least 0.4, preferablyfrom 0.6 to 1.8, particularly preferably from 0.8 to 1.3, ethylenicallyunsaturated groups deriving from component (b21). The reaction betweencomponents (b1) and (b2) is carried out by well-known methods of organicchemistry and is preferably implemented in an organic solvent attemperatures of from 60 to 200° C. The organic solvent employed shouldbe miscible with water. It is also possible to employ a catalyst whichcatalyzes the reaction between epoxide groups and carboxyl groups. Suchcatalysts are well known. Examples are p-toluenesulfonic acid, basicamines, ammonium salts and phosphonium salts.

Component (B2) can be obtained by reacting component (b1) with (b3), amonoester of an ethylenically unsaturated dicarboxylic acid and asaturated fatty alcohol, or a mixture of such monoesters, in a ratiosuch that the reaction product contains not more than 0.25 epoxidegroups and at least 0.4, preferably from 0.6 to 1.8, particularlypreferably from 0.8 to 1.3, ethylenically unsaturated groups derivingfrom Component (b3). Component (B2) is preferably free from epoxidegroups.

Component (b3) can be prepared by simple esterification of anethylenically unsaturated dicarboxylic acid with a saturated fattyalcohol. Examples of ethylenically unsaturated dicarboxylic acids whichcan be employed are maleic acid, fumaric or itaconic acid or theanhydrides of maleic acid, fumaric acid and itaconic acid. The saturatedfatty alcohol employed is preferably a saturated fatty alcohol havingfrom 12 to 26, particularly preferably from 12 to 18, carbon atoms.Examples of fatty alcohols which can be employed are myristyl alcohol,palmityl alcohol, stearyl alcohol, arachidyl alcohol, behenyl alcoholand lignoceryl alcohol.

Component (B3) can be prepared by reacting component (b1) with

(b4) a mixture of components (b21), (b22) and (b3) or a mixture ofcomponents (b21) and (b3) or a mixture of components (b22) and (b3),

the composition of component (b4) and the weight ratio in whichcomponents (b1) and (b4) are reacted with one another being selectedsuch that the reaction product of component (b1) and (b4) contains notmore than 0.25 epoxide groups and at least 0.4, preferably from 0.6 to1.8, particularly preferably from 0.8 to 1.3, ethylenically unsaturatedgroups deriving from component (b21) and/or component (b3). Component(B3) is preferably free from epoxide groups.

The aqueous polymer dispersions provided in accordance with theinvention can be prepared by subjecting 1.0 part by weight of component(A) to free-radical polymerization in the presence of from 0.5 to 9.0,preferably from 1.0 to 5.0, parts by weight of component (B1) or in thepresence of from 0.5 to 9.0, preferably from 1.0 to 5.0, parts by weightof component (B2) or in the presence of from 0.5 to 9.0, preferably from1.0 to 5.0, parts by weight of component (B3) and converting theresulting reaction product, during or after the neutralization of atleast 20 percent of the acid groups contained in the reaction product,into an aqueous polymer dispersion.

The polymerization can be carried out in an organic solvent or solventmixture in the presence of at least one free-radical initiator. Theorganic solvents and free-radical polymerization initiators which can beemployed are those which are the organic solvents and free-radicalpolymerization initiators which are well known for solutionpolymerization.

Solvents which can be used are butylglycol, 2-methoxyropanol, n-butanol,methoxybutanol, n-propanol, ethylene glycol monoethyl ether, ethyleneglycol monoethyl ether, ethylene glycol monobutyl ether, diethyleneglycol monomethyl ether, diethylene glycol monoethyl ether, diethyleneglycol diethyl ether, diethylene glycol monobutyl ether and3-methyl-3-methoxybutanol.

Examples of polymerization initiators which can be used are free-radicalinitiators such as, for example, benzoyl peroxide,azobisisobutyronitrile and t-butyl perbenzoate.

The polymerization is preferably carried out at a temperature of from 80to 160° C., particularly preferably at from 120 to 160° C.

After the end of the polymerization the resulting polymerization productis converted, during or after the neutralization of at least 20 percentof the acid groups contained in the polymerization product, into anaqueous polymer dispersion. Both organic bases and inorganic bases canbe employed to neutralize the acid groups, with tertiary amines such as,for example, dimethylethanolamine, triethylamine, tripropylamine andtributylamine preferably being employed.

The solids content of the aqueous polymer dispersions provided inaccordance with the invention is usually from 20 to 60% by weight,preferably between 35 and 55% by weight.

The aqueous polymer dispersions according to the invention may be usedfor the production of aqueous coating materials. The aqueous coatingmaterials containing the polymer dispersions according to the inventionmay contain a crosslinking agent. Suitable crosslinking agents are aminoresins containing N-methylol and/or N-methylol ether groups, and blockedpolyisocyanates. It is particularly preferred to employ melamine resinsas crosslinking agents, very particular preference being given to thosemelamine resins which contain per molecule from 4 to 6 methylol groups,with at least 3 of these methylol groups being etherified with ashort-chain alkanol such as, for example, methanol or butanol. Examplesof blocked polyisocyanates which can be employed are polyisocyanatesblocked with methyl ethyl ketoxime.

The crosslinking agents are employed in a quantity such that the ratioof equivalents between the reactive groups present in the polymerizationproduct and the reactive groups present in the crosslinking agent isbetween 0.7 and 1.5.

Depending on the intended application of the coating materials producedfrom the aqueous polymer dispersions provided in accordance with theinvention, the coating materials may contain a large number of otheradditives which are required for the particular intended application,for example pigments, dyes, pigment dispersion auxiliaries, rheologyauxiliaries, UV stabilizers, light stabilizers and the like.

Aqueous coating materials which contain as binder the polymerizationproduct present in the aqueous polymer dispersions provided inaccordance with the invention are particularly suitable for theproduction of aqueous coating materials which are suitable for thefinishing of motor-vehicle bodies. They may be employed in processes forthe finishing of motor-vehicle bodies, in which an electrodepositioncoat, a filler coat and a one-layer pigmented topcoat or a two-layertopcoat consisting of a pigmented basecoat and a transparent topcoat arecoated over one another and are baked in succession in two or morebaking steps, for the production of the filler coat, the one-layerpigmented topcoat, the pigmented basecoat or the transparent topcoat.Using the aqueous polymer dispersions provided in accordance with theinvention it is possible in particular to produce transparent aqueouscoating materials which are very particularly well suited to theproduction of the transparent topcoat in the above-described process forthe finishing of motor-vehicle bodies. Transparent topcoats producedusing these coating materials are distinguished, in comparison with thetopcoats prepared in accordance with EP-A-469,646, by an increasedresistance to yellowing. Moreover, coats which have been produced usingaqueous coating materials based on the polymer dispersions provided inaccordance with the invention have good surface properties, inparticular high acid resistance and scratch resistance.

The examples which follow illustrate the invention in more detail. Allparts and percentages are to be understood as by weight unless expresslystated otherwise.

A) Preparation of Aqueous Polymer Dispersions According to the Invention

Polymer Dispersion I

237.4 g of dimerized fatty acid (Pripol® 1009 from Unilever), 150.8 g of3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexylcarboxylate (AralditeCY-179 from Ciba Geigy), 42.1 g of stearic acid, 10.4 g of methacrylicacid, 82.6 g of 1-methoxy-2-propanol and 0.7 g of Cr (III)2-ethylhexanoate are mixed intensively in a 2-liter reactor withstirrer, thermometer, reflux condenser and two feed vessels. The firstfeed vessel is filled with 70 g of diethylene glycol monobutyl ether andthe second feed vessel is filled with a mixture of 86.8 g of styrene,31.2 g of 2-hydroxypropyl methacrylate, 59.3 g of butyl methacrylate,14.4 g of acrylic acid and 8.3 g of dicumyl peroxide.

Reactor and feed vessels are then flushed with nitrogen, and thecontents of the reactor are heated under nitrogen to 120° C. Thistemperature is maintained for 3 hours. The contents of the first feedvessel are then added and the temperature in the reactor is raised to130° C. After this the contents of the second feed vessel are added overthe course of one hour and the temperature is maintained at 130° C. fora further 3 hours. As soon as the contents of the reactor have cooled to100° C., 14.3 g of N,N-dimethylethanolamine and afterwards, over thecourse of 2 hours, 922 g of deionized water are added.

An aqueous dispersion is obtained having a solids content (1 h 130° C.)of 37% by weight.

Polymer dispersion II

Preparation of a Monoester From an Ethylenically UnsaturatedDicarboxylic Acid and a Saturated Fatty Alcohol

749 parts by weight of a saturated fatty alcohol (Nafol® 12, 14®, CondeaChemie), 370 parts by weight of maleic anhydride and 1.1 parts by weightof hydroquinone monomethyl ether are weighed out and heated to 90° C. ina 4 liter steel reactor fitted with stirrer, thermometer, oil heatingand reflux condenser.

The reaction mixture is held at 90° C. until an acid number ≦189 isreached.

Preparation of Polymer Dispersion II

66.1 parts by weight of the monoester prepared by the proceduredescribed above, together with 237.4 g of dimerized fatty acid (Pripol®1009 from Unilever), 150.8 g of epoxycyclohexylmethyl3,4-epoxycyclohexanecarboxylate (Araldite CY-179 from Ciba Geigy), 11.5g of stearic acid, 184.5 g of 1-methoxy-2-propanol and 0.46 g of Cr(III) 2-ethylhexanoate, are mixed intensively in a 2 liter reactor withstirrer, thermometer, reflux condenser and 2 feed vessels. The firstfeed vessel is filled with 70 g of diethylene glycol monobutyl ether andthe second feed vessel is filled with a mixture of 86.8 g of styrene,31.2 g of 2-hydroxypropyl methacrylate, 59.3 g of butyl methacrylate,14.4 g of acrylic acid and 8.3 g of dicumyl peroxide.

Reactor and feed vessels are then flushed with nitrogen, and thecontents of the reactor are heated under nitrogen to 120° C. Thistemperature is maintained for 3 hours. The contents of the first feedvessel are then added and the temperature in the reactor is raised to130° C. After this the contents of the second feed vessel are added overthe course of one hour and the temperature is then maintained at 130° C.for a further 3 hours. As soon as the contents of the reactor havecooled to 100° C., 14.3 g of N,N-dimethylethanolamine and afterwards,over the course of 2 hours, 922 g of deionized water are added. Adispersion is obtained having a solids content (1 h, 130° C.) of 33% byweight.

B Preparation of Clearcoats According to the Invention

Clearcoat I

80.6 parts by weight of polymer dispersion I are mixed thoroughly with14.2 parts by weight of melamine resin (Cymel 327 from AmericanCyanamid, 90%), 3.55 parts by weight of diethylene glycol monobutylether and 1.65 parts by weight of deionized water. The clearcoataccording to the invention is adjusted to a flow time of 25 s in a DIN 4flow cup.

Clearcoat II

Clearcoat II is prepared like clearcoat I, the only difference beingthat 90.36 parts by weight of polymer dispersion II are employed insteadof 80.6 parts by weight of polymer dispersion I.

C) Preparation of a Comparison Clearcoat

76.4 parts by weight of a dispersion prepared in accordance withEP-A-469 646, Example 13, are mixed thoroughly with 14.2 parts by weightof melamine resin (Cymel 327 from American Cyanamid, 90%), 355 parts byweight of diethylene glycol monobutyl ether and 5.85 parts by weight ofdeionized water. The comparison clearcoat is adjusted to a flow time of25 s in the DIN 4 flow cup.

D) Preparation of 2-layer Topcoats

A commercially available, aqueous basecoat containing aluminum pigmentis sprayed at an atmospheric humidity of about 65% onto steel panelscoated with a commercially available electrodeposition coating materialand a commercially available filler, the coatings are flashed off for 5minutes at room temperature, pre-dried at 60° C. for 15 minutes and thencoated over with the clearcoats prepared in accordance with B) and C)(dry film thickness 40-45 μm). After a further drying time of 5 minutesat room temperature and 10 minutes at 80° C. the basecoats andclearcoats are baked together in a circulating-air oven at 150° C. for30 minutes. Afterwards the finishes produced in this way are overbakedonce more for 30 minutes at 140° C. and for 30 minutes at 155° C.Subsequently the yellowing index, which is a measure of the yellowingcaused by the overbaking, is determined in accordance with DIN 6167. Theresults can be taken from the following table:

Finish produced with

    ______________________________________    Yellowing index after              Comparison    overbaking at                 Clearcoat I                            Clearcoat II                                       clearcoat    ______________________________________    140° C.                 1.60       1.42       2.13    155° C.                 2.20       2.00       4.60    ______________________________________

The finishes produced using the clearcoats according to the inventionhave consequently been yellowed to a lesser extent by the overbakingthan the finish produced with the comparison clearcoat.

We claim:
 1. An aqueous polymer dispersion, comprising an at least 20%neutralized, free-radical polymerized product of A and B in a ratio ofone part A to from 0.5 to 9.0 parts B by weight; whereinA consistsessentially of from 2.5 to 13% by weight of at least one acidgroup-containing, ethyleneically unsaturated monomer and from 87 to97.5% by weight of at least one ethylenically unsaturated monomer thatis free from acid groups, and further wherein B is a reaction product ofat least one compound containing at least 1.5 epoxide groups permolecule on average and at least one member of the group consisting of:(1) mixtures of ethylenically unsaturated monocarboxylic acids,saturated fatty acids, and monoesters of ethylenically unsaturateddicarboxylic acids and saturated fatty alcohols; and (2) mixtures ofsaturated fatty acids and monoesters of ethylenically unsaturateddicarboxylic acids and saturated fatty alcohols; and still furtherwherein B contains not more than 0.25 epoxide groups and at least 0.4ethylenically unsaturated groups per molecule on average.
 2. An aqueouspolymer dispersion according to claim 1, wherein the polymerization ofalone would result in a polymer having an acid number of from 23 to 100,a hydroxyl number of from 30 to 200, and a glass transition temperatureT_(g) of from -40 to +70° C.
 3. An aqueous polymer dispersion accordingto claim 1, wherein the compound containing at least 1.5 epoxide groupsper molecule on average is a reaction product ofn mol of at least onecompound that contains on average per molecule from 1.5 to 2.5 epoxidegroups, and n-1 mol of at least one dicarboxylic acid having from 2 to40 carbon atoms, wherein n is 2-10.
 4. An aqueous polymer dispersionaccording to claim 1, wherein the ethylenically unsaturatedmonocarboxylic acid is selected from the group consisting of acrylicacid, methacrylic acid, and mixtures thereof.
 5. An aqueous polymerdispersion according to claim 1, wherein the saturated fatty acid hasfrom 14 to 24 carbon atoms.
 6. An aqueous polymer dispersion accordingto claim 1, wherein the monoester is a monoester of maleic acid, fumaricacid, or itaconic acid and a saturated fatty alcohol having from 12 to26 carbon atoms.
 7. An aqueous polymer dispersion according to claim 1,wherein the reaction product B contains on average per molecule from 0.6to 1.8 ethylenically unsaturated groups.
 8. Aqueous coating materialscomprising a polymer dispersion according to claim
 1. 9. A process forfinishing a surface, comprising the steps of applying, in successivelayers, an electrodeposition coat, a filler coat, and a one-layerpigmented topcoat or a two-layer topcoat consisting of a pigmentedbasecoat and a transparent topcoat; and baking the applied layers insuccession in two or more baking steps, wherein at least one member ofthe group consisting of the filler coat, the one-layer pigmentedtopcoat, the pigmented basecoat, and the transparent topcoat comprisesan aqueous coating material according to claim
 8. 10. A processaccording to claim 9, wherein the transparent topcoat comprises anaqueous coating material according to claim
 8. 11. An aqueous polymerdispersion according to claim 2, wherein the polymer would have an acidnumber of from 40 to 70, a hydroxyl number of from 40 to 140, and aglass transition temperature T_(g) of from -20 to +50° C.
 12. An aqueouspolymer dispersion according to claim 5, wherein the saturated fattyacid has from 16 to 20 carbon atoms.
 13. An aqueous polymer dispersionaccording to claim 6, wherein the saturated fatty alcohol has from 12 to18 carbon atoms.
 14. An aqueous polymer dispersion according to claim 7,wherein the reaction product B contains on average per molecule from 0.8to 1.3 ethylenically unsaturated groups.
 15. A process according toclaim 9, wherein the layers are applied to at least a portion of asurface of a motor vehicle.